fix: nil apiresponses.FailureResponse should not panic

Ideally folks should not create a nil apiresponses.FailureResponse, but
it is possible, and when that happens, calling methods on the nil
apiresponses.FailureResponse should not panic.

Relates to #252

domain/apiresponses/failure_responses.go

@@ -33,7 +33,7 @@ func NewFailureResponse(err error, statusCode int, loggerAction string) *Failure
 // ErrorResponse returns an interface{} which will be JSON encoded and form the body
 // of the HTTP response
 func (f *FailureResponse) ErrorResponse() interface{} {
-	if f.emptyResponse {
+	if f == nil || f.emptyResponse {
 		return EmptyResponse{}
 	}
 
@@ -46,7 +46,7 @@ func (f *FailureResponse) ErrorResponse() interface{} {
 // ValidatedStatusCode returns the HTTP response status code. If the code is not 4xx
 // or 5xx, an InternalServerError will be returned instead.
 func (f *FailureResponse) ValidatedStatusCode(logger lager.Logger) int {
-	if f.statusCode < 400 || 600 <= f.statusCode {
+	if f == nil || f.statusCode < 400 || 600 <= f.statusCode {
 		if logger != nil {
 			logger.Error("validating-status-code", fmt.Errorf("Invalid failure http response code: 600, expected 4xx or 5xx, returning internal server error: 500."))
 		}
@@ -57,11 +57,19 @@ func (f *FailureResponse) ValidatedStatusCode(logger lager.Logger) int {
 
 // LoggerAction returns the loggerAction, used as the action when logging
 func (f *FailureResponse) LoggerAction() string {
+	if f == nil {
+		return ""
+	}
+
 	return f.loggerAction
 }
 
 // AppendErrorMessage returns an error with the message updated. All other properties are preserved.
 func (f *FailureResponse) AppendErrorMessage(msg string) *FailureResponse {
+	if f == nil {
+		return &FailureResponse{error: fmt.Errorf("%s", msg)}
+	}
+
 	return &FailureResponse{
 		error:         fmt.Errorf("%s %s", f.Error(), msg),
 		statusCode:    f.statusCode,
@@ -71,6 +79,14 @@ func (f *FailureResponse) AppendErrorMessage(msg string) *FailureResponse {
 	}
 }
 
+// Error returns the error message
+func (f *FailureResponse) Error() string {
+	if f == nil {
+		return "Error() called on uninitialized apiresponses.FailureResponse"
+	}
+	return f.error.Error()
+}
+
 // FailureResponseBuilder provides a fluent set of methods to build a *FailureResponse.
 type FailureResponseBuilder struct {
 	error

domain/apiresponses/failure_responses_test.go

@@ -108,4 +108,30 @@ var _ = Describe("FailureResponse", func() {
 			Expect(failureResponse.LoggerAction()).To(Equal("log-key"))
 		})
 	})
+
+	Describe("nil", func() {
+		// Because a FailureResponse is public, it's possible to create a nil one.
+		// If someone does that, we should not panic.
+		var e *apiresponses.FailureResponse = nil
+
+		It("allows Error() to be called", func() {
+			Expect(e.Error()).To(Equal("Error() called on uninitialized apiresponses.FailureResponse"))
+		})
+
+		It("allows AppendErrorMessage() to be called", func() {
+			Expect(e.AppendErrorMessage("ok")).To(MatchError("ok"))
+		})
+
+		It("allows LoggerAction() to be called", func() {
+			Expect(e.LoggerAction()).To(BeEmpty())
+		})
+
+		It("allows ValidatedStatusCode() to be called", func() {
+			Expect(e.ValidatedStatusCode(nil)).To(Equal(http.StatusInternalServerError))
+		})
+
+		It("allows ErrorResponse() to be called", func() {
+			Expect(e.ErrorResponse()).To(Equal(apiresponses.EmptyResponse{}))
+		})
+	})
 })